The present invention is primarily, but not solely, intended for applications of production of electrical energy from underwater currents in seas and water-courses. Sea currents can be caused by tidal water, temperature or saline differences, the Coriolis force of the rotation of the earth such as, e.g., the Gulf Stream or of level difference, e.g., on each side of sounds.
The flow velocity of the water in sea currents is usually fairly moderate, i.e., typically just a few m/s and may amount to 5 m/s at the most. The flow velocity in unregulated watercourses is normally of the same magnitude but may in rapids increase up to 10-15 m/s.
Presently, the energy source offered by underwater currents is very little utilized for the generation of electricity. This depends above all on difficulties to achieve economically competitive technology for the same. In this connection, one of the problems is the low flow velocity. The utilization of energy from such flows entails that turbines having low number of revolutions and high torque have to be used.
In these conditions, an expedient turbine type is such a one that has axially directed blades. Conventionally, such a one is formed with the blades thereof connected to a shaft via substantially radially directed stays, which shaft in turn is connected to the rotor of the generator. In applications when a turbine of relatively large dimensions is required, the stresses on the structure carrying the blades become large. This may involve risk of distortions and oscillations in constructions, which may lead to deteriorated geometry of the setting of the blades with accompanying deteriorated efficiency. This may be especially sensitive if the water flow that drives the turbine is unstable, i.e., that fluctuations occur in the velocity and direction of the flow. This may be the case, e.g., in certain types of underwater currents where wave motions on the surface and tidal water affect a flow being stable in other respects. Furthermore, the requisite load-carrying structure makes the turbine complicated and ungainly.
The object of the present invention is to obviate or at least reduce these problems.